Two sides, each consisting of at least one player, compete against each other in a typical sport played with an object, such as a ball, which moves above a playing surface and often impacts the surface. Exemplary sports include tennis and basketball. The playing surface, referred to as a court, consists of an inbounds (“IB”) playing area and an out-of-bounds (“OB”) playing area demarcated by boundary lines. When the object impacts the OB area, the side that caused the object to go out of bounds is typically penalized. In tennis, a point is awarded to the other side. In basketball, possession of the basketball is awarded to the other side. Decisions as to whether the object impacts the playing surface in or out of bounds are often difficult to make for impacts close to the boundary lines.
Additionally, the IB area typically contains internal lines that place certain requirements on the sport. For instance, a tennis court contains three internal lines which, together with the tennis net and a pair of the boundary lines, define four servicecourts into which a tennis ball must be appropriately served to avoid a penalty against the server. It is often difficult to determine whether a served tennis ball impacting the playing surface close to one of these lines is “in” or “out”. Each half of a basketball court usually has a three-point line. At least one shoe of a player shooting the basketball must contact the court behind the three-point line immediately prior to the shot with neither of the shooter's shoes touching the court on or inside the three-point line as the shot is taken for it to be eligible for three points. It is likewise difficult to determine whether this requirement is met when the shoes are close to the three-point line.
Returning to tennis, FIG. 1 illustrates the layout of playing surface 20 of a standard tennis court with line width somewhat exaggerated. For singles, playing surface 20 consists of rectangular IB playing area 22 and OB playing area 24 edgewise surrounding IB playing area 22 and extending to court boundary 26. Singles IB playing area 22 is defined inwardly by two opposite equal-width parallel straight baselines 28 and two opposite equal-width parallel straight singles sidelines 30 extending between baselines 28. Tennis net 32 is situated above a straight net line, usually imaginary but potentially real, extending parallel to baselines 28 substantially midway between them and extending lengthwise between and beyond singles sidelines 30 for dividing singles IB area 22 into two singles half courts.
Singles IB area 22 contains (i) two opposite equal-width parallel straight servicelines 34 situated between baselines 28 and extending lengthwise between singles sidelines 30 at equal distances from the imaginary or real net line and (ii) straight centerline 36 extending lengthwise between servicelines 34 at equal distances from singles sidelines 30. Lines 30, 34, and 36 in combination with the imaginary/real net line, and thus effectively net 32, define inwardly four equal-size rectangular services courts 38. Lines 28, 30, and 34 define two equal-size rectangular backcourts 40.
Playing surface 20 for doubles consists of IB playing area 42 and OB playing area 44 edgewise surrounding IB playing area 42 and extending to court boundary 26. Doubles IB playing area 42 is defined inwardly by baselines 28 and opposite equal-width straight doubles sidelines 46 located outside singles IB area 22. The imaginary/real net line situated below net 32 extends lengthwise between and beyond doubles sidelines 46 for dividing doubles IB area 42 into two doubles half courts. Net 32 extends fully across IB area 42 and into OB area 44. Rectangular doubles alleys 48 extend along doubles sidelines 46 outside singles sidelines 30. FIG. 2 is a less-labeled version of FIG. 1 in which roughly elliptical items 50, of somewhat exaggerated size, represent examples of areas where tennis balls, including just-served tennis balls, contact playing surface 20 and which are variously so close to the tennis lines that it may be difficult to make decisions, referred to as “line calls”, on whether the balls are “in” or “out”.
Players and tennis officials variously make line calls in tennis depending on the availability of officials. Numerous devices, including camera-based devices, have been investigated to assist in making line calls. One notable camera-based device is the Hawk-Eye system in which a group of video cameras in conjunction with a computer track moving tennis balls to provide simulations of their trajectories and predictions of their court contact areas. See Geiger, “How Tennis Can Save Soccer: Hawk-Eye Crossing Sports”, Illumin, 25 Mar. 2013, 3 pp. FIG. 3 illustrates an example of simulated trajectory 60 of tennis ball 62 tracked with Hawk-Eye on one stroke. FIG. 4 depicts simulated contact area 64 of ball 62 near a sideline 30 on another stroke. As FIG. 4 indicates, Hawk-Eye provides a visual notification specifying whether ball 62 is in or out.
The Hawk-Eye simulations are displayed on a screen at which players (and officials) look to see the line calls. This disrupts play. As a result, Hawk-Eye is used for only certain line calls. In particular, officials initially make all line calls with each side allocated a small number of opportunities to challenge official-made calls per set provided that a challenge opportunity is retained if an official-made call is reversed. The use of challenges is distracting to the players. Hawk-Eye's accuracy depends on the accuracy of the predictive data analysis for the simulations and on Hawk-Eye's alignment to the tennis lines, assumed to be perfectly straight even though they are not perfectly straight. Hawk-Eye appears to occasionally make erroneous calls as discussed, e.g., in “Hawk-Eye”, Wikipedia, en.wikipedia.org/wiki/Hawk-Eye, 18 Jul. 2013, 8 pp. While Hawk-Eye has gained high recognition among the camera-based devices, it is desirable to have a better device than Hawk-Eye or any other camera-based device for making line calls.
Line-calling systems utilizing tennis balls with special electrical or chemical treatments have been proposed as, e.g., disclosed in U.S. Pat. Nos. 4,109,911 and 7,632,197 B2. However, such systems are disadvantageous for various reasons. Erosion along the outside of a specially treated tennis ball as it contacts the tennis court and racquets may detrimentally affect the ball's ability to provide the information needed to appropriately communicate with the line-calling system. The electrical or chemical treatments may so affect the bounce characteristics that some tennis players are averse to using specially treated balls. Players and officials are generally unable to rapidly verify the accuracy of the calls.
The possibility of using piezochromic material in making line calls has been raised. A piezochromic material changes color upon applying suitable pressure and returns to the original color upon releasing the pressure. In Bradley, “Interview with Williams James Griffiths”, Reactive Reports, June 2006, 3 pp., Griffiths proposes a thin device to be laid on a tennis court and to contain piezochromic material that changes color upon being impacted by a tennis ball. Griffiths mentions that (i) the piezochromic material would have to be shielded from ultraviolet radiation because piezochromic materials are ultraviolet sensitive and most tennis courts are outdoors and (ii) piezochromic materials generally undergo reverse color change too quickly for a person to check an impact location. Ferrara et al., “Intelligent design with chromogenic materials”, J. Int'l Colour Ass'n, vol. 13, 2014, pp. 54-66, similarly proposes that electrochromic paint be applied at and near the lines of a tennis court for assistance in making line calls and that the same paint could be used for basketball, volleyball, and squash courts.
Tennis players are usually close to baselines 28 during much of a tennis match. The players' shoes would likely cause color changes near baselines 28 in a tennis court using the piezochromic material of Griffith or Ferrara et al. Shoe-caused color changes would sometimes partially or fully overlap ball-caused color changes and thereby degrade the ability of using ball-caused color changes in making line calls.
Charlson et al., International Patent Publication WO 2011/123515, discloses a “piezochromic” device, perhaps better described as an electrowetting device, which changes color in response to a force. One embodiment is a sports tape for determining whether a tennis ball is in or out. Other devices using pressure/force sensing have been investigated for assistance in making line calls as disclosed in, e.g., U.S. Pat. Nos. 3,415,517, 3,982,759, 4,365,805, 4,855,711, and 4,859,986. Line-calling devices using other technologies have also been investigated as, e.g., described in “Electronic line judge”, Wikipedia, en.wikipedia.org/wiki/Electronic_line_judge_(tennis), 19 Jun. 2012, 3 pp. These other line-calling devices are impractical for one reason or another. It is desirable for tennis and other sports needing fast line calls to have a practical line-calling device or system which overcomes the disadvantages of prior art line-calling systems.